The reason for deviation from Mendel's dihybrid cross in T.H. Morgan's experiment is ________.

Question

Prepp Team · Accepted Answer

Understanding Genetic Deviation in Morgan's Dihybrid Cross

The question asks about the reason why T.H. Morgan's dihybrid crosses showed results that deviated from the expected ratios based on Mendel's principles. Mendel's dihybrid cross experiments led to the principle of independent assortment, predicting a 9:3:3:1 phenotypic ratio in the F2 generation when considering two traits controlled by genes on different chromosomes or far apart on the same chromosome.

T.H. Morgan conducted experiments using the fruit fly, Drosophila melanogaster. He studied the inheritance of several traits, including eye color, body color, and wing size. When performing dihybrid crosses involving certain traits, such as body color (yellow vs. brown) and eye color (white vs. red), Morgan observed that the phenotypic ratios in the F2 generation were significantly different from the expected 9:3:3:1 Mendelian ratio. Instead, he observed a much higher proportion of parental phenotypes and a lower proportion of recombinant phenotypes.

Why Morgan's Results Deviated from Mendel's Dihybrid Cross

The key concept that explains this deviation is linkage.

Linkage refers to the phenomenon where genes located close together on the same chromosome tend to be inherited together as a unit.
Mendel's principle of independent assortment holds true when genes are on different chromosomes or located far apart on the same chromosome, allowing them to assort independently during meiosis.
However, when genes are located close together on the same chromosome, their alleles tend to stay together during gamete formation. This reduces the chance of new combinations of alleles (recombinants) appearing in the offspring.

Morgan's experiments with Drosophila provided evidence that genes are located on chromosomes and that genes on the same chromosome can be linked. The degree of linkage depends on the distance between the genes on the chromosome. Tightly linked genes show very little recombination, while loosely linked genes show more recombination due to crossing over.

Analyzing the Options

Let's look at why the other options are not the primary reason for the deviation from the Mendelian dihybrid cross ratio observed by Morgan in this specific context:

Pleiotropy: This is when a single gene influences multiple distinct phenotypic traits. While important in genetics, pleiotropy explains the effect of one gene on many traits, not the altered inheritance pattern of two different genes compared to Mendel's ratios.
Overlapping: This term is not a standard genetic concept explaining deviations from Mendelian inheritance patterns in the context of dihybrid crosses. It might refer to phenotypic expression, but not the mechanism of gene inheritance causing deviation from expected ratios.
Polygenic Inheritance: This involves multiple genes contributing to a single trait (e.g., human height or skin color). It explains continuous variation in traits, not the deviation in the inheritance pattern of distinct traits like those studied by Morgan in these specific crosses.

Therefore, the reason for the deviation from Mendel's dihybrid cross expectations in T.H. Morgan's experiment involving certain linked genes was linkage.

Comparing Mendelian Independent Assortment and Linkage

Here's a brief comparison of how independent assortment (Mendel's observation) and linkage affect dihybrid cross results:

Feature	Mendel's Independent Assortment	Morgan's Linkage
Gene Location	On different chromosomes OR far apart on the same chromosome.	Close together on the same chromosome.
Assortment during Meiosis	Alleles of different genes assort independently.	Alleles of linked genes tend to be inherited together.
Gamete Types	Four types of gametes produced in equal proportions (if parents are heterozygous).	Parental type gametes produced in higher proportion than recombinant type gametes.
F2 Phenotypic Ratio (starting from heterozygous parents for both traits)	Typically 9:3:3:1 (for complete dominance).	Deviates from 9:3:3:1, with higher proportion of parental phenotypes.

Revision Table: Key Concepts

Term	Definition	Relevance to Question
Mendel's Dihybrid Cross	Studying inheritance of two traits simultaneously; predicts independent assortment if genes are unlinked.	Provides the expected baseline (9:3:3:1 ratio) that Morgan observed deviation from.
Independent Assortment	Alleles of different genes separate independently during gamete formation.	Mendel's principle, contradicted by linked genes.
Linkage	Genes on the same chromosome inherited together.	The direct reason for deviation from Mendelian ratios in Morgan's experiments.
T.H. Morgan	Geneticist who used Drosophila; discovered sex-linkage and gene linkage.	His experiments provided crucial evidence for linkage.

Additional Information on Genetic Linkage

The concept of linkage discovered by Morgan was a major advancement in genetics. It showed that inheritance patterns are not always as simple as predicted by independent assortment, especially when genes reside on the same chromosome. The closer two genes are on a chromosome, the stronger the linkage and the less likely they are to be separated by crossing over during meiosis. This allowed Morgan and his students to map the relative positions of genes on chromosomes based on the frequency of recombination (crossing over) between them.

Crossing over, also known as recombination, is an event during meiosis where homologous chromosomes exchange segments. This exchange can separate alleles of linked genes, leading to recombinant gametes. The frequency of crossing over between two linked genes is roughly proportional to the distance between them. This forms the basis of genetic mapping.